Electroconductive tin oxide

ABSTRACT

Electroconductive doped tin (IV) oxide is produced in bulk by combining a first material which is selected from: 
     a) a thermal precursor of a dopant element; 
     b) a tin (II) compound comprising a dopant element; and 
     c) a mixture of materials each falling within (a) or (b); 
     with a second solid material which is tin (II) oxide or a thermal precursor thereof, heating the combined materials and oxidising the heated combined materials, wherein the dopant element is or comprises phosphorus, boron, tungsten, tantalum or niobium. Halogen may also be present. Preferably the combined materials are in solid form, and the oxidation step includes atmospheric oxidation at an elevated temperature. The conductive oxide may be used in a non-consumable electrode, or as a filler for a polyolefin, for example.

FIELD OF THE INVENTION

This invention relates to the preparation and use of electroconductivedoped tin oxide, and particularly to the preparation and use ofelectroconductive doped tin oxide in bulk form.

BACKGROUND OF THE INVENTION

Doped tin oxide is an example of a semiconductor. The lattice of thelatter can accommodate ions of different valencies. Such ions are notnecessarily of similar size to tin (IV) or oxygen (II). The relativeconcentration of the differing ions can be used to control theelectrical conductivity.

It is well known to dope tin oxide to produce and electricallyconductive material. Thus for example antimony doped films have beencommonly used as transparent conductive films for light modulatingdevices and displays, and there is also a considerable body ofliterature relating to the formation of fluoride doped films. However,references to the production of electroconductive doped tin oxide inbulk, for example as a powder, as opposed to thin films, are rare.

While at first this may appear strange, it should be remembered thatsimultaneous co-precipitation of anions is rarely, if ever, possible.When forming a film, conditions may be such that the simultaneousthermal decomposition of dopant and tin containing chemicals, uponcontact with a heated substrate, increases the chances of dopant beingincorporated into the tin oxide lattice. Examples of the reactionsemployed for film formation are: BuSnCl₃ +Sn(BF₄)₂ ; Bu₂ Sn(OBu)₂ +CF₃CO₂ H; SnCl₄ +NH₄ F; and MeSnCl₃ +HF.

DESCRIPTION OF THE PRIOR ART

A method of producing fluoride doped tin oxide is disclosed in EP 0 448946 in which a tin (II) oxide or oxide precursor is intimately mixedwith tin (II) fluoride and heated in an oxidising atmosphere to producefluoride doped tin (IV) oxide.

Common dopants for tin oxide are antimony and fluoride. These sufferfrom a number of disadvantages, not least that the starting materialsare toxic and unpleasant. In addition, fluoride doped tin oxides aretemperature sensitive and lose fluoride and electrical conductivity whenexposed to high temperature. One alternative dopant which shows promiseis phosphorus. However, to date the only technique reported forproducing phosphorus doped tin (IV) oxide, as disclosed in U.S. Pat. No.4,514,322, involves milling metastannic acid with elemental phosphorus.This is clearly not a desirable operation.

One method for producing a fluorine doped tin oxide material in bulkform is disclosed in Japanese Patent Application HEISEI 2-197014,published 3 Aug., 1990 (filing no. HEISEI 1-17196). This describes thetreatment of stannic oxide with fluorine gas in an inert gas diluent,preferably at elevated temperatures (300° to 600° C.). Clearly a processinvolving fluorine gas, particularly at elevated temperatures, hasattendant difficulties, and one of our aims was to develop a simplerprocess than this.

Another method is described in EP 0 441 427 in which a non-aqueoussolution of a tin (IV) salt is combined with an aqueous fluoride, theresulting co-precipitated hydroxide materials being dried and heated toat least 600° C., preferably under nitrogen. Whilst the fluoride couldbe tin (II) fluoride, there is no teaching that air oxidation of astannous component could be beneficial.

European Patent Application 0 235 968 A discloses a process in which anaqueous medium containing a tin (II) carboxylate and a dopant materialis treated with hydrogen peroxide to provide a transparent solution.After shaping (for example, formation of a membrane on a substrate) orconversion to a powder, e.g. by spray drying, this material is calcinedto produce conductive tin oxide. Clearly in such a case oxidation byhydrogen peroxide is a "wet" process at relatively low temperatures, ascompared to the relatively high temperature oxidation of a solidmaterial in embodiments of the present invention.

SUMMARY OF THE INVENTION

The present invention provides a method of producing anelectroconductive doped tin (IV) oxide comprising producing a mixture bycombining a first material which is selected from:

a) a clean thermal precursor of a dopant;

b) a tin (II) compound comprising a dopant; and

c) a mixture of materials each falling within (a) or (b);

with a second material which is tin (II) oxide or a clean thermalprecursor thereof, heating the combined materials and oxidising theheated combined materials, wherein the dopant is or comprises an elementother than a halogen.

A convenient way of effecting the oxidising step is by exposure to anoxidising atmosphere such as air.

The dopant, other than halogen, may comprise any species which iscapable of being incorporated into the tin (IV) oxide lattice to producea stable compound with, for example, low electrical resistance, or arelatively sensitive photoconductivity. In the lattice the dopantspecies may be considered to be cationic or anionic. A mixture of two ormore dopant species can be used to advantage.

For the avoidance of confusion, in the following description and claims,elements are designated according to a version of the periodic table inwhich the transition elements, e.g. Sc to Zn, are in groups IIIa to IIb,and the remaining, non-transition, elements, such as P and B, fallwithin groups IIIb to IIa.

A suitable dopant may be or comprise a Group Vb element, a particularlysuitable species being P(V); or it may be or comprise a Group IIIbelement such as boron. It may also comprise or be a member of thetransition metal series, for example, a Group Va or VIa element. It mayalso comprise a halogen, such as fluorine, for example, in combinationwith any of the aforementioned species.

By "a second material which is tin (II) oxide and/or a clean thermalprecursor thereof" is meant:

(i) tin(II) oxide itself, for example as a dry powder;

(ii) a material which will cleanly provide tin(II) oxide when subjectedto heat in a non-oxidising atmosphere and will not introduce foreigncations or anions into the system which would remain despite thesubsequent heating and exposing operations. Typical examples arestannous carboxylates such as acetate and formate and stannoushydroxide; and

(iii) mixtures of (i) and (ii).

Tin(II) oxide in a physical form other than the dry powder, for examplean aqueous slurry of the powder may be considered to fall within (i) or(ii).

By "clean thermal precursor of the dopant" is meant a material capableof cleanly transferring or providing dopant to the second material, atleast when heated, and which does not introduce foreign cations oranions into the system which would remain despite the subsequent heatingand exposing operations. For example, phosphoric acid, phosphorous acidor ammonium orthophosphate could be used to provide a phosphorus dopant.Mixed phosphorus/fluoride dopant species could be provided by the use offluorophosphates, for example.

Tin is a cation which would remain. However it is not foreign to thesystem and so is acceptable. Therefore tin (II) materials similar to the"clean thermal precursors" can be used, as "tin (II) compoundscomprising the dopant", for example tin (II) fluoride for a fluorinedopant.

With first material(s) in liquid form, for example, an aqueousphosphoric acid, the resulting process is a "wet" chemical process, andthis itself has attendant disadvantages, for example the need to dry thecombined first and second materials.

However, both first and second materials can be solids, which enables a"dry" reaction to be carried out. A preferred solid first material isdiammonium orthophosphate.

It is believed that at least in some cases the method is mosteffectively performed with three stages of progressive heating. A firststage of heating may be necessary to drive to completion an initial stepof preparing a solid intermediate material which consists essentially oftin (II) ions, oxide ions and ions providing the dopant, and/or fordrying this solid intermediate materials. The second stage of heating isbelieved to allow the ions providing the dopant to move and assume amore uniform distribution. The third stage of heating provides atemperature at which oxidation of the solid intermediate tin(II)containing materials to the electroconductive doped tin oxide product iseffective.

Nevertheless, we have found that the process is fairly robust, and it isnot always necessary to distinguish between these three heating stages,nor is it necessary to confine exposure to the oxidising atmosphere tothe third stage, as will be seen from the detailed description. Thusthere need be no pause between the first stage and the second stage,and/or between the second stage and the third stage, and, while theoxidising atmosphere is necessary during the third stage, it can bepresent at the second or even the first stage. In some cases the initialreaction at ambient temperature may be sufficient to produce the solidintermediate material containing tin(II) ions, oxide ions, and dopantions, thus rendering the first stage optional (for example to drive thereaction further).

According to a second aspect, the invention provides a method ofproducing electroconductive doped tin oxide comprising the steps ofheating a solid material consisting essentially of tin (II) ions, oxideions, and ions providing the dopant, and oxidising the heated materials.

The method according to the second aspect may further comprise aninitial step of preparing the said solid materials, said initial stepincluding combining a first material which is selected from:

a) A clean thermal precursor of the dopant;

b) A tin(II) compound comprising the dopant;

c) A mixture of materials which fall within (a) or (b);

with a second material which is tin(II) oxide or a clean thermalprecursor thereof.

The first and second materials will be the same as those employed forthe method according to the first aspect of the invention, and theinitial step may in some cases, particularly where the combining is a"wet" reaction, require a drying stage to give the solid materials.Again a convenient way of performing the oxidising is by exposure to anoxidising atmosphere such as air.

It will be noted that, as with the method according to the first aspect,the method according to the second aspect also uses a starting materialin which the tin is essentially present as tin (II), in contrast withmost of the prior art discussed above.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention will now be described in more detail(Examples 1 to 8), together with a comparative Example 1 not formingpart of the present invention. It is to be understood that the scope ofthe invention is defined by the appended claims and is not limited bythese examples.

EXAMPLE 1

90 gm of stannous oxide was mixed with 20 gm of diammoniumorthophosphate in a Turbula mixer. The mixture was placed in a coveredalumina pot and heated to 200° C. for 17 hrs then 300° C. for 2 hrs,400° C. for 1.5 hrs and finally 500° C. for 18 hrs. The contents of thealumina pot were removed and lightly ground to mix them. A sample waspressed at 19 MPa and found to have a volume resistivity of 24 ohm cm.The phosphorus content was estimated to be approximately 4% by weight.

A sample of the powder was further heated to 1000° C. for 4 hrs andvolume resistivity measured as before. A value of 63 ohm cm was obtainedshowing that phosphorus doped stannic oxide is relatively insensitive tohigh temperature.

Comparative Example 1

The experiment described above was repeated using stannic oxide in placeof stannous oxide. The resulting powder was an electrical insulator.

EXAMPLE 2

90 gm of stannous oxide was slurried with 100 ml water and 17.5 gm 85%orthophosphoric acid added with constant stirring. The slurry was driedat 110° C., homogenised, and calcined at 500° C. for 4 hrs in an opencrucible. This experiment was repeated using different quantities oforthophosphoric acid with the following results:

    ______________________________________                                                       Volume Resistivity                                             Amount of Acid at 19 MPA                                                      g              Ohm cm                                                         ______________________________________                                        17.5           45                                                             1.74            6                                                             .18            20                                                             3.5             3                                                             ______________________________________                                    

EXAMPLE 3

17.5 gm of 85% orthophosphoric acid were added dropwise to 90 gm ofstannous oxide with continuous mixing. The damp solid was homogenised ina Turbula mixer and then calcined in a covered alumina pot at 200° C.for 17 hrs followed by 500° C. for 5.5 hrs. The powder was remixed andfurther calcined at 500° C. for 4 hrs. The resulting powder had a volumeresistivity of 12 ohm cm when pressed at 19 MPa.

EXAMPLE 4

Using the techniques of example 1 a tungsten doped tin oxide wasproduced using 6.3 gm of ammonium tungstate and 90 gm of stannous oxide.The final powder had a volume resistivity of 8 ohm cm at 19 MPa.

EXAMPLE 5

Using the techniques of example 1 a boron doped tin oxide was producedusing 2.2 gm of boric acid and 90 gm of stannous oxide. The final powderhad a volume resistivity of 2.4 ohm cm at 19 MPa.

EXAMPLE 6

The experiment of example 5 was repeated with the addition of 0.74 gm ofstannous fluoride to the mixture before calcining. The product had avolume resistivity of 1.77 ohm cm at 19 MPa.

EXAMPLE 7

Using the techniques of Example 1,a tantalum doped tin oxide wasproduced using 7.87 gm of tantalum oxide and 90 gm of stannous oxide.The final powder had a volume resistivity of 196 ohm cm at 19 MPa.

EXAMPLE 8

Using the techniques of Example 1,a niobium doped tin oxide was producedusing 4.74 gm of niobium oxide and 90 gm of stannous oxide. The finalpowder had a volume resistivity of 743 ohm cm at 19 MPa.

What is claimed is:
 1. A method of producing an electroconductive dopedtin (IV) oxide wherein electroconductivity is imparted by the presenceof a dopant which comprises phosphorus, boron, tungsten, tantalum,niobium or a mixture thereof, comprisingproducing a mixture bycombining, optionally in the presence of water, a first material whichis selected from the group consisting of:a) a dopant material whichcomprises the dopant, or a precursor thereof; b) a tin (II) compoundcomprising the dopant or a precursor thereof; and c) a mixture ofmaterials each falling within (a) or (b); with a second solid materialwhich is tin (II) oxide or a precursor which on heating yields tin (II)oxide, heating the combined materials including, if water is present,drying the combined materials, and substantially completely oxidizingthe heated and dry combined materials, to provide said electroconductivedoped tin (IV) oxide.
 2. A method according to claim 1 wherein the firstmaterial is solid.
 3. A method according to claim 1 wherein the dopantcomprises phosphorous.
 4. A method according to claim 1 wherein thedopant is phosphorous.
 5. A method according to claim 1 wherein thedopant also comprises at least one halogen.
 6. A method according toclaim 1 wherein the oxidising is effected by exposure to an oxidisingatmosphere.
 7. A method according to claim 6 wherein the oxidisingatmosphere is air.
 8. A method according to claim 6 wherein saidexposing to said oxidising atmosphere occurs only when the combinedmaterial has been heated.
 9. A method according to claim 6 wherein saidexposing occurs throughout said heating.
 10. A method of producingelectroconductive doped tin (IV) oxide comprising the steps ofheating adry solid material consisting essentially of tin (II) oxide, and dopantmaterial which comprises phosphorus, boron, tungsten, tantalum, niobiumor a mixture thereof, and oxidizing the heated material to a degreesufficient to provide conversion of substantially all tin (II) to tin(IV).
 11. A method according to claim 10 and further comprising aninitial step of preparing said solid material, said initial stepincluding combining said dopant material with a solid second materialwhich is tin (II) oxide, a precursor which on heating yields tin (II)oxide or a mixture of said tin (II) oxide and said precursor which onheating yields tin (II) oxide.
 12. A method according to claim 11wherein the dopant material is solid.